The present invention relates to voltage regulators, and in particular, to wideband voltage regulators for use with microprocessor, microcontrollers or other like high-frequency devices in which suppression of current transients is desired.
As the speed of high performance microprocessors increases, consistent with CMOS transistor feature size reductions, the required power supply voltage continues to shrink. Further, the increased load and higher processor speed results in more severe current transients on the microprocessor""s power supply. For example, as microprocessors execute instructions, particularly at faster rates, severe transients can occur. These severe current transients, if not properly regulated, can cause noise on the power supply that can induce errors in the microprocessor.
Typically, extensive decoupling techniques, wherein capacitors are placed across the load between the supply and ground references, in combination with active voltage regulation, are used to supply the instantaneous charge required by the microprocessor under dynamic operation. On-chip decoupling, e.g., on the integrated die, generally takes excessive chip area and reduces reliability. Off-chip decoupling typically has limited effectiveness because of the parasitic inductance in the power supply leads. In addition, off-chip as well as on-chip active voltage regulation employing conventional circuit design approaches lacks the bandwidth to respond to fast load transients and typically has limited effectiveness because of the parasitic inductance in the power supply leads.
A conventional wideband voltage regulator 100 is illustrated in FIG. 1, wherein VIN is an input power supply, VOUT is a regulated output supply, VCC is a voltage source required to bias the regulator circuitry, and which may be common with input power supply VIN, VREF is a voltage reference which determines the regulated output voltage supply VOUT, and an amplifier 104 that comprises an output buffer amplifier having a gain G. During operation, load current supplied to regulated output supply VOUT is primarily drawn from the input power supply VIN. In addition, a closed loop differential transconductance amplifier 102 formed by transistors Q1 and Q2 senses the difference between output voltage VOUT and reference voltage VREF and, through its feedback arrangement, strives to minimize the difference between the two voltages, VOUT and VREF.
The effectiveness of a conventional wideband voltage regulator, such as regulator 100, to respond to fast load transients is primarily a function of the small signal bandwidth of regulator 100, the output impedance of output buffer amplifier 104 and the large signal slew rate performance of regulator 100. With reference to FIGS. 2 and 3, which illustrates the response to fast load transients of regulator 100, under severe fast load transients, e.g., state xe2x80x9cBxe2x80x9d in FIG. 2, input transconductance amplifier 102 formed by Q1 and Q2 can fully switch, and the reaction response of the voltage regulator 100 can then be initially limited by the large signal slew rate performance of amplifier 102. Under this condition; the response of amplifier 102 becomes xe2x80x9cslew rate limitedxe2x80x9d and the rate of change of the output voltage can react no faster than the ratio of I0/(2*CCOMP). However, simply increasing the amplifier quiescent current (I0) will not allow improvement of the slew rate, since doing so will raise the open loop gain of amplifier 102 by the same factor. Thus, an equivalent increase in the compensation capacitor (CCOMP) is required to achieve the same closed loop phase margin. Moreover, other techniques such as emitter degeneration, or the use of low gm FET devices, will slightly improve slew rate performance but at the cost of open loop gain required for regulation accuracy.
Accordingly, a need exists for a wideband voltage regulator which overcomes the problems of high current transients, and does not have the limitations of the prior art with respect to slew rate and regulation accuracy.
The method and circuit according to the present invention addresses many of the shortcomings of the prior art. In accordance with various aspects of the present invention, a wideband voltage regulator is provided which can provide suppression of fast transients. In accordance with an exemplary embodiment, a voltage regulator can include a boosting circuit and a sensing circuit. The boosting circuit can be suitably configured to boost the voltage regulator response, while the sensing circuit can determine when such a boost may be desired. Accordingly, the response of the voltage regulator can be accelerated to a fast load transient beyond the closed loop bandwidth limited response or the slew rate limited response of the voltage regulator.
In accordance with various exemplary embodiments, an exemplary voltage regulator can be configured with an active sensing circuit comprising a sensing amplifier with switch control outputs, and a boosting circuit comprising N stored charge sources and (3Nxe2x88x921) switches that are configured to accelerate the voltage regulators response to a fast load transient beyond the closed loop bandwidth limited or slew rate limited response of the voltage regulator. The stored charge sources can comprise various components, such as boost capacitors, additional power supplies, or actively biased devices.
In accordance with another aspect of the present invention, the sensing circuit can be configured in various manners. In accordance with an exemplary embodiment, a method for determining when the sensing circuit can switch the state of the stored charge sources can comprise a comparison of the output voltage of the regulator to a constant reference voltage. In accordance with another exemplary embodiment, the sensing circuit can switch the state of the stored charge sources by comparing the voltage drop across the parasitic inductance between the voltage regulator output and the load to a constant reference voltage, or by comparing the difference between the voltage drop across the parasitic inductance of the supply side and the ground return of the load. Further, the sensing circuit can be triggered on a one-shot basis with preset pulse width, or by any other suitable trigger methodology. Still further, a differential offset voltage can be added to the sense amplifier to suitably adjust or configure the sensitivity of the sensing circuit.